Brevipalpus

Many growers of Pleiones have worried about the possible presence on their plants of a pest mite called  Brevipalpus oncidii, one of the so-called false spider mites. Though known about for many years, we should thank David Harberd for focusing recent attention on it in his article in The Bulletin of the Alpine Garden Society (Vol. 66 No. 4  pages 480-483). His description however of the effects this pest can have on our plants may have depressed many of us and even put off some people from growing Pleiones altogether. In another article (Vol. 67 No. 2 pages 178-179) Ian Butterfield reminded us that poorly growing plants are not always due to Brevipalpus and of the importance of good cultural techniques. Where Brevipalpus is present though it can seriously affect the growth of our Pleiones. However I have good news - Brevipalpus can be easily and effectively controlled by several means.  

About Brevipalpus oncidii  

Brevipalpus belongs to a family of mites known as the Tenuipalpidae or false spider mites. Knowledge of the biology and control of this family of mites is still limited and fragmentary, though more is known about those species which are important pests of agricultural and horticultural crops. They are a particular problem on citrus, tea, grapes and various ornamental plants. The species causing problems on Pleiones is Brevipalpus oncidii which in commercial terms is regarded as an occasional pest, recorded mainly on Oncidium and Odontoglossum species in California and England . Although we know it also infests Pleiones, I can find no published research about it on this genus, probably because they are not of sufficient commercial value to warrant spending money on scientific studies.

 My observations

Prompted by Dr. Harberd’s article and the lack of other available information, I decided to make observations on Brevipalpus oncidii and to try out various possible control methods. I have some entomological training and access to a microscope with an attached camera, so I started by looking at Brevipalpus under the microscope to familiarise myself with it and to take photographs. I then placed several infested bulbs in a warm place (my airing cupboard!) to culture the pest and watch the life cycle. The temperature varied between 20 and 24 centigrade throughout the period.

 a) The eggs

Brevipalpus eggs are more or less spherical and a bright orangey-red. They were laid on all parts of the Pleione plants - roots, pseudobulbs, bulbils and leaves. However they obviously had a preference for some locations over others : they most favoured laying on the underside of the pseudobulbs and in the depression at the very top of the old pseudobulbs formed where last years leaf used to be attached. The time from laying to hatching varied somewhat, but most hatched about 20 days after laying

 b) The adults

After hatching, the young mites go through some juvenile stages and take from 4 to 6 weeks to reach the adult stage. The adults are oval in shape, narrowing at the rear end and are reddish in colour. They walk over all parts of the plant, though often favouring the underside of the pseudobulbs. Their movements are rather slow compared to those of the more familiar Red Spider Mite Tetranychus urticae, but they move more quickly the warmer they are. They feed by inserting their mouthparts into the cells of the plant, injecting a kind of saliva to digest the contents and then sucking out the resulting fluid. The saliva contains chemicals which include persistent systemic toxins. These toxins are transported to all parts of the plant and are responsible for the detrimental effects seen on our Pleiones.

 Experiments with control measures

I tried treating Brevipalpus with various chemicals using dip tests - literally dipping infested pseudobulbs (during their dormant season) into chemical solutions, immersing them for just a second before removing them. The treated bulbs were then examined at regular intervals under the microscope to see if the mites were dead. Examinations continued for a month after treatment in order to ascertain if any eggs hatched or whether they had also been killed by the treatment. One treatment was subsequently also tested by spraying growing plants in the summer.

 Results of experiments

David Haberd has mentioned the effectiveness of the chemical Childion and my own tests confirm this, but this is a professional pesticide unavailable to amateurs. He also suggested as an extreme measure to start a collection afresh from bulbils which he believed were a part of the plant not infested by the mites. Unfortunately my own observations on Brevipalpus revealed that they happily infest all parts of a plant including the bulbils and I have taken photos of them on these. I therefore cannot recommend this approach. Happily, it is not necessary as there are now effective chemical controls available to amateurs.

 Based on the tests, I can recommend the following treatments for control of Brevipalpus mites:

 a) Bifenthrin

Bifenthrin is the active ingredient in some pesticides available to amateurs. This was used at the strength recommended for control of Red Spider Mites. It effectively killed adult Brevipalpus but had no effect on the eggs. To gain complete control therefore, more than one treatment is needed (in order to kill mites hatching from the unaffected eggs after the first spray). The studies on life cycle suggested a suitable programme would be to treat three times with the treatments timed three weeks apart. This assumes the warm temperatures the studies were done at. To test this theory and also to see if Bifenthrin works when applied as a spray during the growing season (when temperatures may be expected to be closer to those in the studies), it was tried on a collection known to be well infested with Brevipalpus. The plants were sprayed thoroughly at the end of June, again in the third week of July and a final time three weeks later, in the second week of August. At the end of the year the bulbs were examined while dormant under the microscope. No Brevipalpus mites or eggs could be found.

 b) Neem Oil

Neem is a natural product extracted from the Neem tree and has been used in Asia as a natural insecticide for hundreds of years. It works by a variety of actions, both physical and chemical. I believe Neem is not approved as an insecticide in the UK so strictly speaking should not be used for this purpose and is not sold (as far as I know) any longer as such. My tests were done while it still was sold as an insecticide. Using this at the standard strength recommended on the label, it effectively killed both adults and eggs. As it is a natural product this would be a good choice for those of you who prefer an organic alternative. This was only tested by the dipping method but there is no reason to think it would not also be effective as a spray.

 c) Horticultural Spraying Oil

The value of light mineral oils as insecticides has been known for some time. Their action is not due to being toxic to the pests but is a physical effect, clogging the breathing pores of the insects and thus smothering them. The product I tested was “Hortichem Spraying Oil”. Although a professional product, it can be purchased and used by amateurs. This is because it was recently de-registered (due to its physical, non-toxic mode of action). This means it is no longer regulated by the Control of Pesticides Regulations 1986. The producers of the product confirmed this to me in a recent Email and enquiries at a local stockist confirmed they are happy to sell it to the general public. In my tests it was, like the Neem oil, very effective at killing both eggs and adults of Brevipalpus.

 d) Childion

This chemical is for professionals only, but for them I can confirm that this product is effective at killing both adults and eggs of Brevipalpus.

 e) Methylated Spirit

A rather extreme treatment which some have tried is to soak the dormant bulbs in methylated spirit. This treatment is probably illegal under UK regulations, but for historical interest it might be noted that a soak of an hour or longer was needed to kill both adults and eggs. The obvious question is whether this might also adversely affect your bulbs? I know of two people who have tried this without any adverse effect - including one who soaked for 24 hours! The bulbs still grew away fine afterwards. I have however also heard of one person who says his bulbs died after this treatment so beware....!

 f) Physical removal

For those who have just a few bulbs and access to a microscope, it is possible to use a needle to pick off the mites from each bulb and squash the eggs. You may miss one or two but this method is quite effective at keeping infestation levels down to almost undetectable levels. It is of course time consuming but is at least a completely chemical-free option for those who grow just a few Pleiones.  

None of the chemicals described showed any harmful effects to the plants when used as recommended.

 Conclusions

Brevipalpus need not be viewed as being any worse a problem than other pests which commonly affect our plants. It should certainly be no reason to put people off from successfully growing and enjoying Pleiones. The mites can be easily and successfully controlled by the methods described. My personal routine is to dip the bulbs while dormant in Spraying Oil and then spray with Bifenthrin as described during the growing season. It is particularly important to treat newly acquired bulbs once your own collection is free of the pest as my observations confirm that this pest is very widespread. I always check new bulbs under the microscope and have found that Brevipalpus is almost always present on bulbs received from a wide variety of sources. Adopting the control methods described should I believe keep your plants free of this pest.

 Suppliers in the UK :

Bifenthrin is widely available at Garden Centres sold under various product names – look for “active ingredient: Bifenthrin” on the pack.  

Home